The invention relates to correcting the weft distortions which occur in fabrics during different working cycles. These distortions are eliminated, usually before the fabric is dried by employing various design straightening devices, often with automatic control.
In general, the straightening devices can be divided into two categories: passive--without sensing the angle of weft distortion, where the straightening device is driven by passing fabric, and active--where the straightening device is driven by auxiliary drives in accordance to signals from weft distortion sensing devices called weft sensors.
Work principle of the first category relies on stretching the fabric in the lateral direction with as little as possible longitudinal tension, thus all kinds of weft distortion including bow distortion can be removed, but not completely. Non-stretching, loose weave fabrics can be straightened, and other fabrics with a dense weave and elastic fabrics sometimes need to be overcorrected to get rid of the distortions permanently. Most straightening devices from the second category employ bow rolls for removing bow distortions.
They can remove only distortions which are symmetrical in respect to the middle of the web, but fail to remove partial bow distortions. Other machines using curved cylinders or plurality of axially extending, pivotally mounted flexible rods (disclosed in U.S. Pat. No. 4,768,265) can better handle unsymmetrical web distortions, but they are not popular because of their large dimensions and many moving parts.
It is an object of the present invention to improve the apparatus employing bow rolls, so it would be suitable for removing bowed weft distortions in the fabric, especially partial bow distortion, often called a hook.